[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[via cfe-commits]

trcrsired wrote:

> 2. 'rewrite the world', which is what MinGW, Cygwin, et al have done, but require dragging along an entirely separate sysroot, which may diverge from the vendor in features, correctness, and behaviour, meaning libraries like https://github.com/microsoft/wil do not compile with a MinGW toolchain

But it will get easily compiled with windows-msvc-sysroot which is exactly the point here.
https://github.com/trcrsired/windows-msvc-sysroot

https://github.com/llvm/llvm-project/pull/96417